The field of the invention is systems and methods for magnetic resonance imaging (“MRI”). More particularly, the invention relates to systems and methods for high-resolution MRI.
Slice dithered enhanced resolution simultaneous multislice (“SLIDER-SMS”) MRI has recently been used in diffusion MRI scans to resolve sub-millimeter slices, as described in co-pending PCT Application Serial No. PCT/US15/53719. Acquiring sub-millimeter slices is impractical with conventional slice selection due to the low SNR and constraints on practical RF pulse duration. SLIDER-SMS overcomes these limitations by selecting a thick slab and introducing sub-voxel shifts along the slice direction between acquisitions. Thin slices are then obtained using a super-resolution technique during image reconstruction, providing slice images that are thinner than the originally excited slabs.
As compared with serial acquisition from individual thin slices, SLIDER provides higher SNR because the spins from a thick-slice are always contributing to the signal at any moment in time. The high signal level of each thick-slice acquisition is advantageous in diffusion imaging, where it permits accurate removal of background phase, thereby providing real-valued diffusion images (assuming minimal through-slice dephasing).
A drawback of the SLIDER approach, however, is that the shifted thick-slices do not form an orthonormal encoding basis. This causes noise amplification during the image reconstruction process that must be suppressed using regularization, which in turn blurs the slice profiles of the final high resolution slices. Therefore, a tradeoff exists between noise level and spatial resolution due to the linear dependence in the slice encoding functions.